Self-diffusion of n-pentane

Self-diffusivity of n-Pentane and n-Hexane and their Mixtures in Silicalite-1... 

Fig. 17 Self-diffusivity of n-pentane in silicalite-1 at various temperatures and loadings...

Figure 20 shows the self-diffusivities of n-pentane and n-hexane as a function of gas mixture composition. The loadings of both components depend on the gas phase composition fraction. Note that in these experiments the total hydrocarbon pressure is kept constant (6.6 kPa). The loading of a feed of pure n-hexane under these conditions (433 K, 6.6 kPa) is 3.6 molecules per unit cell. Unexpectedly, we observe a lower diffusion coefficient for pure n-pentane than for n-pentane in a mixture with n-hexane. Tentatively, we ascribe this to the more drastic increase in n-hexane dif-fusivity with loading than for n-pentane. As discussed earlier, this can be ascribed to stronger repulsive interactions for the longer hexane hydrocarbons. It is clear from Fig. 20 that at low n-hexane concentrations, its diffusion is slower than of n-pentane. The diffusivity of hexane increases with n-hexane loading, while the diffusivity of pure n-pentane was found to be independent on the concentration at this temperature. Repulsion between n-pentane... 

Around a value of the gas-phase fraction of 2-methylpentane of about 0.83, the influence of the acid sites on the n-hexane diffusivity is not dominant anymore in comparison to the pore occupation of slow-diffusing 2-methyl-pentane. Figure 14 shows the dependence of the diffusivities of both components versus the concentration of adsorbed 2-methylpentane in terms of molecules per unit cell. The diffusivities of n-hexane in silicalite-1 and H-ZSM-5 become nearly equal when the concentration of 2-methylpentane reaches approximately 2.75 molecules per unit cell. For 2-methylpentane we And that the self-diffusivity in silicalite-1 becomes very close to the value in H-ZSM-5 at the same loading. 

Karger and co-workers (47,48) have developed a novel technique to directly measure pore diffusion in supercritical media using pulsed field gradient NMR. Their results suggest that the critical point of the fluid, n-pentane, within the pores is about 30 K lower than in the bulk fluid. Near the pore critical point the self-diffusivity of the fluid in the pores increases, then plateaus near the Knudsen diffusion limit (dependent on pore size). 

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